We have demonstrated the elaboration of collagenase by two human breast carcinoma lines in continuous culture. The protease was present in a latent form and was activated by short exposure to trypsin or plasminogen. Such zymogen forms of collagenase have not been previously reported from malignant tissues in continuous culture. The chick allantoic membrane was successfully used as a rapid bioassay to differentiate between invasive and non-invasive tumors. A relative alteration in collagen metabolism in transformed vs. non-tumorigenic cells was reflected in their dose response to hydroxyproline analogues as monitored by cell growth. A study of cell lines derived from explants of adult connective tissue confirmed that some phenotypic alterations of cells can be correlated with their tumorigenic potential in vivo.